Equipment for treating wastewater by anaerobic ammonium oxidation includes an anaerobic ammonium oxidation vessel to receive water to be treated containing ammonium and nitrite and to denitrify ammonium and nitrite in the water by anaerobic ammonium oxidizing bacteria, forming denitrified treated water, an acclimatization vessel downstream from the anaerobic ammonium oxidation vessel to receive the denitrified treated water from the anaerobic ammonium oxidation vessel, the acclimization vessel containing an immobilizing material that collects the anaerobic ammonium oxidizing bacteria in the denitrified treated water as immobilized microorganisms attached to the immobilizing material, wherein the acclimatization vessel has a structure such that the immobilizing material is prevented from flowing out of the acclimatization vessel, and a pipe connecting the acclimatization vessel to the anaerobic ammonium oxidation vessel.